A high efficiency particulate filter is a relatively expensive filter which is often used in a breathing apparatus or respirator to capture very small diameter particulate matter. During use, however, such a filter often becomes quickly laden with larger diameter particulate matter. This larger size matter, however, can be successfully filtered by a less expensive pre-filter whereas its entrapment in the high efficiency filter significantly shortens the life of this costly device.
U.K. Patent No. 2,070,965 discloses a cup-shaped cover for a respirator filter. The filter is a single unit and includes a chemical filter and a particulate filter. It does not show a removable pre-filter positioned in front of a high efficiency particulate filter.
U.S. Pat. No. 4,771,771 discloses a pre-filter located in front of a chemical cartridge filter. The pre-filter, however, is fixed on the respirator and cannot be replaced to extend the life of the main filter.
U.S. Pat. No. 4,179,274 discloses a removable filter secured between a chemical cartridge and a cover. The cover, however, has an open grid face that allows air to be drawn directly through the face of the cover. As a result, this cover cannot provide the removable filter and the chemical cartridge with much protection against damage.
It would be desirable, therefore, to provide a removable cover assembly having an inexpensive pre-filter which is located in front of a high efficiency particulate filter to trap the larger particulate matter thereby preserving the more expensive filter. Further, it would be desirable if the pre-filter could be quickly and easily removed and exchanged as it becomes clogged. Finally it would be desirable to provide a cover assembly which protects the pre-filter as well as the high efficiency particulate filter from damage during use.